Numerous means have been sought to improve the fuel-efficiency of moving bodies and, especially, moving bluff bodies by reducing their aerodynamic drag. In the field of surface transportation, and particularly in the long-haul trucking industry, even small improvements in fuel efficiency can reduce annual operating costs significantly. It is therefore advantageous in the design of a vehicle to reduce drag forces, thereby increasing the aerodynamic properties and efficiency of the vehicle.
The over-the-highway cargo-hauling tractor-trailer combination is one vehicle that experiences excessive aerodynamic drag. Generally described, tractor-trailer combinations typically include a tractor having a so-called fifth wheel by which a box-like semi-trailer may be attached to the tractor by an articulated connection for transportation of the cargo trailer. Conventional tractors may include tandem (dual) or tridem (triple) rear axles, where two or three sets of axles are positioned close together along the length of the tractor, near the fifth wheel, to carry the weight of the trailer. Conventional trailers also may include tandem or tridem axles.
It is well known that bluff bodies, such as tractors and trailers, contribute significantly to aerodynamic drag, as evidenced by the formation of a wake along the length of and in the trailing region behind the trailer. Additionally, various components of the tractors and trailers, such as the wheels and tires, also contribute significantly to the aerodynamic drag of the vehicle. The generation of eddies can be contributed to the shape and rotation of the wheels and tires as the vehicle is driven down the road, especially at freeway speeds. Airflow around the rotating wheels and tires forms eddies, creating significant turbulence and increasing drag to reduce vehicle efficiency. The net result is the creation of considerable aerodynamic drag.